I've got to respectfully disagree on this point. Rivets do loosen up over time and use, but that doesn't mean the boat is unsafe. A few micrometers of flex generally won't work harden the aluminum panels as there's just not enough flex to do so.There have probably been more starcraft rebuilds than any other recreational brand, and that's because they can be rebuilt on a budget and be just as safe as when they were new. The splashwell structure will crack if they were run with a rotten transom, and the chine rail can crack if the side panels were soft and it was a pre hull-stiffener boat. The splashwell cracks are an easy fix with backer and jb weld. A chine rail crack is bad and extremely unsafe. But it's not that common. Some people add their own stiffeners:
If you have the deck out I'd do a garden hose leak test, mark the leaky rivets and replace them with new solid rivets. Gluvit will work too, but it's more of a band aid fix or a precautionary step while you have the hull interior open.
I would stay away from spray-in foam and use closed cell rigid foam insulation panels. Same flotation benefit but cheaper and it won't lead to waterlogged foam in a few years.